GB2444942A

GB2444942A - Reaction member of a brake adjuster having a plastic ring gear moulded thereon

Info

Publication number: GB2444942A
Application number: GB0625547A
Authority: GB
Inventors: Fra Giovanni
Original assignee: Automotive Products Italia SV SRL
Current assignee: Automotive Products Italia SV SRL
Priority date: 2006-12-21
Filing date: 2006-12-21
Publication date: 2008-06-25
Also published as: GB0625547D0

Abstract

A brake adjuster (26) for adjusting the working clearance of the shoes of an automotive drum brake has a first end element having a head portion (25) for engagement with a first brake shoe and a threaded stem portion (29). The adjuster also has an intermediate element (28) in threaded engagement with the first end element (25,29) and a second end element having a head portion (27) for engagement with a second brake shoe and a stem portion slideably connected with the intermediate element (28) and free to rotate relative thereto. A reaction member (30) is mounted on the intermediate element(28) and a toothed ring gear (35) of plastics material is moulded onto the intermediate element (28) between the second end element and the reaction member. A leaf spring (31) acts between the second end element (27) and the reaction member (30) for pushing the second end element away from the intermediate element. The leaf spring (31) carries a pawl (36) designed to engage the teeth (35a) of the ring gear (35) for rotation of the ring gear to adjust the effective length of the adjuster.

Description

BRAKE ADJUSTERS

This invention relates to brake adjusters, hereinafter referred to as of the kind described, for adjusting the working clearance of the shoes of an automotive drum brake, the adjuster including three co-axial elements: a first end element having a head portion for engagement with a first brake shoe and a threaded stem portion; an intermediate element in threaded engagement with the first end element; a second end element having a head portion for engagement with a second brake shoe and a stem portion slideably connected with the intermediate element and free to rotate relative thereto; a reaction member mounted on the intermediate element; a toothed ring gear for rotation with the intermediate element and axially located between the second end element and the reaction member; and a leaf spring acting between the second end element and the reaction member for pushing the second end element away from the intermediate element, said leaf spring carrying a pawl designed to engage the teeth of the ring gear for rotation of the ring gear to adjust the effective length of the adjuster.

An example of such a brake adjuster is disclosed in the Applicant's published International patent Application No. W003/025415 Al.

Whilst such brake adjusters are well known and are effective in use they have hitherto been relatively expensive to manufacture.

It is an object of the present invention to provide a form of brake adjuster of the kind described which is cheaper to manufacture.

Thus according to the present invention there is provided a brake adjuster of the kind described in which the ring gear is a plastics component which is moulded onto the intermediate element for co-rotation therewith.

Such a construction provides economies of manufacture since the intermediate element can be formed by a cold pressing operation from, for example, large diameter wire stock with the ring member directly moulded thereon from a plastics material such as that sold under the FORTRON trade mark.

Preferably the intermediate element has one or more formations thereon around which the plastics ratchet wheel is moulded to improve the bond between the ratchet wheel and intermediate element.

The invention also provides a method of manufacturing an intermediate element and a toothed ring gear of an adjuster of the kind described comprising forming a metal intermediate element, placing the intermediate element in a mould, injecting plastics material into the mould to surround part of the intermediate element, and form a plastics ratchet wheel thereon allowing the plastics material to solidify, and removing the intermediate element and plastics ratchet wheel from the mould.

A plurality of metal intermediate elements may be placed in individual cavities in a multi-cavity moulding tool for simultaneous moulding of the plastics ring gear onto each respective intermediate element.

The present invention will now be described with reference to the accompanying drawings in which:-Figure us a side view of a vehicle drum brake including a brake adjuster embodying the present invention; Figure 2 is a perspective view of the brake adjuster used in Figure 1; Figure 3 is a perspective view of a spring and pawl used in the adjuster of Figure 2; Figures 4 and 5 show perspective and sectional views respectively of an intermediate element and moulded plastics ring gear in accordance with the present invention; Figure 6 shows a perspective view of the intermediate element of figures 4 and 5 prior to the moulding of the ring gear; Figure 7 shows a perspective view of the form of the moulded ring gear without the intermediate element for clarity, and Figure 8 shows a perspective view of an alternative form of intermediate element.

Referring to figure 1 this shows a drum brake 10 having a pair of brake shoes 11,12 which at their lower ends react against a fixed abutment 13 and at their upper ends are forced apart by an hydraulic wheel cylinder 14. The shoes are kept in contact with the abutment 13 by a lower return spring 15 and in contact with the wheel cylinder 14 by an upper return spring 16.

Thus during normal service operation the shoes are brought into contact with the inner surface 17 of brake drum 18 in the conventional manner by actuating wheel cylinder 14 whose pistons 19 and 20 respectively outwardly displace the contacting ends of shoes 11 and 12 to engage the drum. In the conventional manner the two shoes are held down by hold down clips 21 and 22 respectively. The drum brake is also provided with a handbrake operating lever 23 pivoted at 24 on brake shoe 12.

The hand brake operating lever contacts the right hand first head element 25 of an adjuster in the form of a strut 26 whose left hand second head element 27 contacts the other brake shoe 11. In conventional manner if, for example, the handbrake lever 23 is pivoted on pivot 24 in a clockwise sense as viewed in figure 1 the handbrake lever will push head element 25 of adjuster strut 26 to the left thus pushing the brake shoe 11 to the left. This in effect causes the two brake shoes 11 and 12 to move apart relative to each other and hence again engage the inner surface 17 of the brake drum 18 thus applying the hand brake.

The adjuster strut 26 basically comprises three components. The first head element 25, the second head element 27 and an intermediate element 28. The head element has a threaded stem portion 29 which is received in a screw threaded bore in the right hand end of intermediate element 28. Similarly the second head element 27 has a stem portion which is axially slideable within an internal bore in the left hand end of intermediate element 28. A sheet metal reaction member 30 surrounds the intermediate element 28 and is axially located relative thereto. A bow spring 31 reacts at one end on an abutment 32 provided on the reaction member 30 and at its other end on an abutment 33 provided on a clip like reaction member 34 mounted on the stem portion of left hand head portion 27. A ring gear 35 having teeth 35a is carried on intermediate element 28 is engaged by a pawl 36 which forms part of bow spring 31.

Bow spring 31 biases head element 27 to the left relative to the intermediate element 28. The effect of this is that when the head element 27 is moved into the bore in the intermediate member when the brake shoes are retracted by the springs and 16 this causes the bow spring 31 to assume a more bowed configuration so that the pawl 36 tends to be drawn up out of engagement with the tooth 35a of ring gear 35 which is it is currently engaging. When the amount of movement of the left hand head portion relative to the right hand portion exceeds a predetermined level (indicating a predetermined amount of retraction of the brake shoes which in turn indicates a predetermined amount of wear) then the pawl will engage the next tooth on ring gear 35 and when the brake is next applied the energy stored in the bowed spring rotates the intermediate element 28 slightly relative to the head element 25 to increase the effective length of the adjuster strut 26 to take up the wear.

In accordance with the Applicants earlier PCT Application No. WO 03/025415 Al a temperature sensitive mechanism is provided to ensure disengagement of the pawl 36 from the teeth 35a of ring gear 35 at high temperatures to prevent over adjustment of the drum brake. This feature is provided by forming the pawl 36 as a bi-metal element as shown in figure 3 in which the pawl comprises a first metal layer which is the same metal as the remainder of the bow spring and a second metal layer 41 which extends only over the pawl element of the spring or, alternatively, by forming the pawl as described in the Applicant's earlier PCT Application No. WO 03/25415 Al. With this type of construction when the bow spring heats up as a result of the general hot condition of the brake the pawl will tend to deflect to the left as shown by arrow X thus tending to disengage the teeth 35a of ring gear 35 and prevent over adjustment of the adjuster strut.

Thus far described the adjuster is of a known type.

In accordance with the present invention, the intermediate element 28 of adjuster 26 has the ring gear 35 moulded thereon from plastics material such as that sold under the FORTRON trade mark by Hoechst Aktiengesellschaftrricona GmbH (for example FORTRON 11 3L44) which has the necessary hard wearing and temperature resisting properties required to meet with the operating environment of the adjuster.

The intermediate element 28 is manufactured by a cold pressing operation from a large diameter wire blank of appropriate length. In this pressing operation the bore 54 is formed in the intermediate element 28 together with a pair of circumferentially spaced ribs 55 and a circumferentially extending groove 56 over which the plastics ring gear 35 is moulded to provide for the secure attachment of the ting gear to the intermediate element both axially and radially. As can be seen from figure 7, the plastics ring gear 35 has recesses 55a which are formed around the ribs 55 and a control circumferential band 56a which is formed in groove 56.

Ring gear 35 with its teeth 35a is moulded onto the intermediate element 28 in a multi-cavity injection moulding tool with a separate intermediate element 28 placed in each respective cavity of the mould. The intermediate element has grooves 59 into which the moulding tool extends to seal the gap between the mould and the intermediate element.

The necessary screw thread for engagement with the threaded portion 29 on first end element 25 is cut into bore 54.

Figure 8 shows an alternative form of intermediate element 28 in which the ribs 55 are provided with teeth 57 over which the plastics ring gear 35 is moulded. Teeth 57 may extend completely around ribs 55 or only in small groups as shown. Also the ribs 55 have conical surfaces 58 onto which the mould portions in which ring gear 35 is moulded close. By arranging the mould to close on conical surfaces 58, accurate closing of the mould is ensured which helps to eliminate any clearance between the mould and the intermediate element 28 thus avoiding the formation of plastics burrs which could otherwise impair the function of the clearance adjuster.

As will be appreciated, the above two part construction for the intermediate element 28 enables the element to be constructed more economically and with the use of fewer machining operations.

Claims

1. A brake adjuster of the kind described in which the ring gear is a plastics component which is moulded onto the intermediate element co-rotation therewith

2. A brake adjuster according to claim 1 in which the intermediate element has one or more formations thereon around which the plastics ratchet wheel is moulded to improve the bond between the ratchet wheel and intermediate element.

3. A brake adjuster according to claims 1 or 2 in which the ring gear is moulded from FORTRON material.

4 A brake adjuster according to any one of claims 1 to 3 in which the intermediate element is of generally cylindrical from formed by a cold pressing operation

5. A method of manufacturing an intermediate element and a toothed ring gear of an adjuster of the kind described comprising forming a metal intermediate element, placing the intermediate element in a mould, injecting plastics material into the mould to surround part of the intermediate element, and form a plastics ratchet wheel thereon allowing the plastics material to solidify, and removing the intermediate element and plastics ratchet wheel from the mould.

6. A method according to claim 5 in which a plurality of metal intermediate elements are placed in individual cavities in a multi-cavity moulding tool for simultaneous moulding of the plastics ring gear onto each respective intermediate element.

7. A brake adjuster of the kind described having an intermediate element and plastics moulded ring gear constructed and arranged substantially as hereinbefore described with reference to and as shown in the accompanying drawings.